1. Field of the Invention
This invention relates to interchangeable automatic exposure control apparatus for cameras having electric shutters, and is more particularly directed to interchangeable pentaprism devices employing such apparatus.
2. Description of the Prior Art
Single lens reflex camera having viewfinders constructed to employ different, interchangeable pentaprism devices so that the same camera body can be used for different types of photography are well known.
It has been proposed in the prior art to provide, for mounting on a camera body containing a manually adjustable exposure control circuit, a pentaprism device having a light-metering circuit for providing an electrical signal corresponding to the light intensity and a storage circuit for storing the electrical signal which is present immediately before opening of the shutter, thereby to form a camera having automatic control of the shutter speed. However, for the reasons set forth below, the accuracy of such control heretofore has been unsatisfactory. The manually adjustable exposure time control apparatus contained in such a camera body includes an RC network for providing, upon opening of the shutter, an electrical signal which varies with time in accordance with various manually preset exposure factors, such as the light intensity and the film speed. The control apparatus further includes a switching circuit which de-energizes an electromagnet to release a shutter-closing member when the time-varying electrical signal reaches a predetermined threshold value (i.e., the "trigger level"). However, due to the manufacturing tolerances for the values of the resistor and capacitor which form the RC network, the characteristic of the time-varying signal is not precisely the same for different exposure control circuits. The differences between characteristics of the different time-varying signals have been compensated for by suitable adjustment of the threshold values of the associated switching circuits so that different control circuits provide precisely the same shutter speed under identical exposure conditions. Thus, the threshold values of the switching circuits in different camera bodies are not the same. As a result, when a pentaprism device of the type described above is mounted on different camera bodies having such exposure control apparatus in order to provide cameras in which the time-varying signal is varied automatically in accordance with the light intensity, the shutter speed of the different cameras in the automatic mode will not be the same under identical exposure conditions.
To avoid this problem, it has been proposed to provide the pentaprism device with a voltage divider for setting the threshold value of the switching circuit in the camera body when it is desired to provide automatic exposure control, the camera body being provided with its own voltage divider for setting a different threshold value when it is desired to provide manual exposure control. This method requires a change-over switch of complicated construction for selecting the manual and automatic modes of operation. Moreover, the accuracy of the shutter speed provided by this pentaprism device in the automatic mode is still unsatisfactory, because the trigger level of the switching circuit is determined not only by the threshold voltage applied thereto, but also by the value of the offset voltage of the switching circuit. This offset voltage, which is the minimum voltage difference which can be applied between the two input terminals of the switching circuit to cause the switching circuit to be operated or switched, is determined by the values of the electrical components forming the switching circuit. Due to the manufacturing tolerances for these components, the offset voltage will not be the same for the switching circuits in different camera bodies. Consequently, when a pentaprism device of this type of mounted on different camera bodies, the shutter speed in the automatic mode will not be the same under identical exposure conditions.